1. Field of the Invention
The present invention relates to a semiconductor composite film provided with an organic semiconductor thin film, a method for forming the semiconductor composite film, a thin film transistor including the semiconductor composite film, a method for manufacturing the thin film transistor, and an electronic apparatus.
2. Description of the Related Art
An organic thin film transistor (OTFT) can be formed by a process at temperatures lower than those for an amorphous silicon TFT or a low-temperature polysilicon TFT in the related art and, therefore, is formed on even a flexible plastic substrate having the bendability with no stress. Consequently, development in thin displays and other various new applications is expected. Furthermore, since a substrate can be formed by an inexpensive process, e.g., coating or printing, through the use of a coating material soluble into a solvent without using a vacuum process or photolithography, a reduction in cost is expected.
Regarding pattern printing of an organic semiconductor thin film in OTFT production, the use of a polymer based organic semiconductor material is advantageous from the viewpoint of the printability. For example, regarding the formation of an organic semiconductor thin film layer by an ink-jet method, there are many reports and polymer based organic semiconductor materials, e.g., poly(9,9-dioctylfluorene-co-bithiophene) (F8T2) (refer to C. W. Sele et al., Advanced Materials Vol. 17, p. 997 (2005)) and poly(5,5′-bis(3-dodecyl-2-thienyl)-2,2′-bithiophene) (PQT-12) (refer to A. C. Arias et al., Applied Physics Letters vol. 85, p. 3304 (2004)), have been used.
On the other hand, in consideration of the characteristics of the organic semiconductor thin film itself, the use of low-molecular organic semiconductor material is advantageous. For example, regarding the organic semiconductor thin film formed by applying a spin coating method or other coating methods, in the case where the high-molecular organic semiconductor material is used, the mobility is no more than about 0.1 cm2/Vs. On the other hand, in the case where the low-molecular organic semiconductor material is used, there are many reports in which the mobility exceeds 1 cm2/Vs. J. Jang et al. applied a dichlorobenzene solution of dihexyl quarter thiophene (DH4T) serving as a low-molecular organic semiconductor material by an ink-jet method so as to produce a bottom gate type OTFT and, thereby, achieved the mobility of 0.043 cm2/Vs. However, accurate control of the parameters, e.g., a substrate temperature, is desired, and it is expected that instability of film formation due to a coffee stain phenomenon and the like and characteristic variations and the like resulting therefrom become problems.
Consequently, a technique for pattern-printing an organic semiconductor thin film with good film-quality controllability, even when a low-molecular organic semiconductor material is used, is desired.
Under the circumstances, a polymer blend method has been proposed, in which an organic semiconductor thin film is formed by using a polymer material together with a semiconductor material.
In the polymer blend method, for example, pattern printing is conducted by using an ink in which an insulating polymer material is polymer-blended to a low-molecular organic semiconductor material, and the organic semiconductor material and the polymer material are phase-separated in the film thickness direction. At this time, a technique for facilitating the phase separation through surface energy control of the substrate has been reported, and a thin film transistor operation through the use of the semiconductor thin film layer formed by this technique has been ascertained (refer to Japanese Unexamined Patent Application Publication No. 2005-243822 and Japanese Unexamined Patent Application Publication No. 2006-179905).
Furthermore, regarding such a polymer blend method, it is described that the charge mobility is increased and the stability of the semiconductor thin film layer is improved by polymer-blending a binder resin to a low-molecular semiconductor material of acene base or the like (refer to Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2007-519227).
Regarding the above-described polymer blend method accompanying phase separation, further uniform film formation is expected. That is, in the case of fine droplets formed by, for example, application through ink-jet, Marangoni convection resulting from surface tension distribution occurs during drying of a solvent and, thereby, film formation variation occurs easily because of solute transport. However, the polymer blend method has effects of restricting such solute transport and improving the wettability with the substrate. Consequently, uniform film formation is facilitated.
Moreover, the polymer blend method also has an advantage that the ink viscosity is improved. Therefore, application to pattern printing, in which not only the ink-jet method, but also a high-throughput printing technique, e.g., screen printing or gravure printing, is applied, of an organic semiconductor thin film is also expected.